It is well known to employ permeable membrane to separate or selectively enrich a gas mixture. For example, membranes are used in the separation of H.sub.2 from supercritical gasses including N.sub.2, CO and CH.sub.4 ; the separation of CO.sub.2 and water vapor from natural gas; and the enrichment of air by nitrogen or oxygen. In addition, hydrogen is recovered from ammonia production plants using large scale membrane technology, and, likewise, hydrogen is recovered from coal gassification processes for the production of synthetic fuel.
The fundamentals of gas separation are based upon a permeability equation, which at low pressures in the absence of strong interactions between gas components may be expressed as: EQU .alpha..sub.AB =P.sub.A /P.sub.B
where:
.alpha..sub.AB is referred to as the ideal separation factor; PA1 P.sub.A is the permeability of gas component A in the membrane; and PA1 P.sub.B is the permeability of gas component B in the membrane.
It is generally known that gas separation membranes may be cast from polymers. The separation of gas components by polymer membranes is thought to depend on chemical affinities, kinetic diameters and structural characteristics; it being known generally that rubbery polymers are characterized by high diffusion and relatively low selectivity while glassy polymers are characterized by lower diffusion and higher selectivities.
In any given situation, however, it is presently impossible to engineer or tailor the physical properties (e.g., gas transport properties such as permeability and selectivity) of a polymer membrane with any reasonable degree of confidence. That is, there is currently no reliable and/or convenient method available by which the final properties of a new polymer membrane may be predicted so that a membrane with those desired final properties can then be manufactured. Rather, the properties can only be determined accurately after a membrane is formed from a particular polymer composition and then physically tested for its efficacy to separate a gas (or gasses) of interest.
Thus, although various synthesized copolymer materials are effective for use as gas separation membranes, the gas transport properties of the membranes, e.g., gas permeability and separation, are limited in the sense that they are a direct result of the characteristics of the synthesized copolymer and are not predictable based on the properties and ratios of the monomers utilized. Consequently, &he permeability and selectivity of such membranes can not be readily tailored for specific applications requiring an increase or a decrease in permeability and/or selectivity with respect to certain gasses.
According to the present invention, however, polymer membranes can now be "engineered" to have physical properties tailored for specific end-use applications. Surprisingly, it has now been discovered that certain different polymers with known properties can actually be blended to yield engineered polymer membranes (i.e., membranes having tailored properties for specific end-use applications).
Prior to the present invention, polymer blending has traditionally been thought to be either problematic or of no benefit in the membrane field. Polymer blending has been viewed as problematic because different polymers are generally not miscible with one another. Those few polymers which are thought to be miscible offer no blending advantage in the membrane field because of various reasons, including difficulty in blending, poor mechanical properties, limited range of gas transport properties, and complex relationships between blend composition and gas transport properties.
It has now been discovered, however, that certain polymers form miscible blends and offer all the advantages of ease of preparation, predictability, and the possibility of tailoring gas transport properties over a broad range. The gas transport properties of a membrane resulting from a blend according to the present invention can be predicted based on a linear relationship between the logarithms of the respective permeabilities of the individual components of the blend and the weight percent of those components. This relationship allows facile tailoring of the transport properties of the blend. While this type of semilogarithmic relationship has been described in the literature, it is considered to be a special case (i.e., special in that there can be no extrapolation to other polymer blends), not a general case. Muruganandam and Paul, Gas Sorption and Transport in Miscible Blends of Tetramethyl Bisphenol-A Polycarbonate and Polystyrene. 25 Journal of Polymer Science 2215-29 (1987); and Chiou, Barlow, and Paul, Sorption and Transport of Gases in Miscible Poly(Methyl Acrylate)/Poly(Epichlorohydrin) Blends, 30 Journal of Applied Polymer Science, 1173-86 (1985). In fact, most miscible polymer blends demonstrate significant deviations from the semilogarithmic additivity found in the present invention. Paul, Gas Transport In Homogeneous Multicomponent Polymers, 18 Journal of Membrane Science, 75-86 (1984).
Generally, the present invention provides for gas separation membranes made from a miscible blend of polyimide polymers, each polymer having different gas transport properties (e.g., gas permeability and selectivity) when individually formed into a membrane. Because of the miscibility of certain categories of polyimide polymers, permeability and gas selectivity of a resulting blend can be customized to best suit particular applications by selecting the appropriate polymer ratio of each component in the blend to yield optimum permeability and selectively for a particular application. In particular, customized membranes prepared from the blends of this invention are extremely useful in separation processes involving, for example, H.sub.2, N.sub.2, CH.sub.4, CO, CO.sub.2, He, and O.sub.2, by virtue of the fact that the polyimide blends exhibit high permeability and selectivity.
As used herein and in the accompanying claims (and as will be appreciated by one of ordinary skill in the art), the term "membrane" or "membranous structure" refers, without limitation, to structures used in separation applications, for example, thin films, thick films, composite structures, asymmetric structures, hollow fibers, modules, and like items.